Thresher Maw

Thresher Maws are gigantic subterranean worms. These creatures can grow up to 90 meters long. These worms lie beneath the surface and attack targets by surprise from below. They can be found on numerous worlds but are mostly native to Tuchanka, and must be defeated as a rite of passage for adolescent Krogans. Actually defeating a Thresher Maw is not required to pass the rite of passage however and merely fighting it until a certain amount of time is enough to pass. The last person to have beaten a Thresher Maw (If Grunt didn't do it in his rite of passage) was Urdnot Wrex. It has been confirmed that they will reappear in Mass Effect 3 fighting against the Reapers.

In Mass Effect 3, the largest thresher maw ever seen named Kalros appears on Tuchanka and manages to destroy a Reaper.